Invited Speaker： Prof. He YAN

Introduction：Dr. He Yan (Henry) obtained his Ph.D. in Chemistry from Northwestern University with Professor Tobin Marks. After that, Dr. Yan spent most of his research career at Polyera Corporation, a leading company in the Organic Electronics industry. In 2009, Dr. Yan and his team published the first high-mobility n-type semiconducting polymer in Nature and the work was referred to as the "new transistor age" on the cover page of Nature. With the achievement, Dr. Yan was invited to receive the IdTechEx Printed Electronics "Best Organic Material" award in Dresden, Germany in 2010. After returning to HKUST, Dr. Yan’s group achieved world record efficiency organic solar cells multiple times. Dr. Yan’s group has published several important papers on fullerene and non-fullerene OSCs including two papers on Nature Energy and two papers on Nature Comm. in the past two years. The paper Nature Comm (2014, 5, 5293) has been cited over 1250 times in two years, making it by far the most cited paper among over 3000 Nature Communication articles published in 2014.  

In 2016, Prof. Yan’s team developed a record-efficiency organic solar cell, which was put on the renowned “Best Research-Cell Efficiencies Chart” by the National Renewable Energy Laboratory of the United States, which records all the best efficiency cells around the world over the past 40 years. Shortly after this publication, Prof. Yan team had another important breakthrough by discovering a new material system that demonstrated ultrafast and efficient charge separation despite a nearly zero charge separation driving force, meaning that the more environmentally-friendly OSCs may be able to perform as good as inorganic solar cells in the future. This research work led to a “back-to-back” publication at Nature Energy.

Prof. Yan was also elected the chair of the 2016 Gordon conference on Hybrid Electronic and Photonic Materials and Phenomena and 13th International Symposium on Functional Pi-Electron Systems in 2017 at HKUST. Besides having outstanding achievements in research, Prof. Yan also showed great dedication to teaching and has been awarded School of Science Teaching award in 2016, as well as Research Award in 2015.  

【Lecture Title】Temperature dependent aggregation enables efficient organic solar cells with fullerene or non-fullerene acceptors

-- A new path toward next generation organic solar cells  

Time:  08:50-10:00 am, Apr. 12th, 2017
Location:  New MSE Building, No. 01 meeting room
Abstract:  Organic solar cell (OSC) technology has attracted much attention due to its promise as low-cost conversion of solar energy. Despite recent progress, several limitations are holding back OSC development. For instance, current high-efficiency (>9.0%) OSCs{Liao, 2013 #103} are restricted to materials combinations that are based on limited donor polymers and only one specific fullerene acceptor, PC71BM. Furthermore, best-efficiency OSCs are mostly based on relatively thin (100 nm) active layers. Thick-film OSCs generally exhibit lower fill factors and efficiencies compared to the best thin-film OSCs. Here we report multiple cases of high-performance thick-film (300 nm) OSCs (efficiencies up to 10.8%, fill factors up to 77%) based on conventional PCBM and many non-PCBM fullerenes. Our simple aggregation control and materials design rules allowed us to develop, within a short time, three new donor polymer, six fullerenes (including C60-based fullerenes), and over ten polymer:fullerene combinations, all of which yielded higher efficiency than previous state of art devices (~9.5%). The common structural feature of the three new donor polymers, the 2-octyldodecyl (2OD) alkyl chains sitting on quaterthiophene, causes a temperature-dependent aggregation behavior that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. This results in a well-controlled and near-ideal polymer:fullerene morphology (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregation during warm casting and thus insensitive to the choice of fullerenes.  

For the second part of the talk, I will discuss non-fullerene OSCs. For the well-developed OSCs technology, one of the most important factors that limit the efficiency of OSCs is the relatively large voltage loss from the bandgap (Egap) of the absorber to the open circuit voltage (Voc) of the cell. The large voltage loss in high-efficiency OSCs is due to two main factors. One is relatively large non-radiative recombination loss in OSCs, evidenced by extremely low electroluminescence quantum efficiency (EQEEL) of OSC blends (typically in the range of 10-6-10-8). The other is the existence of a significant offset between the bandgap of the donor/acceptor materials and the energy of the CT state (Egap –ECT). Here we report a non-fullerene OSC ( based on a novel polymer named PffBT3T-E,A and a SMA named SF-PDI2) that has a negligible driving force yet exhibits fast and efficient charge separation. The solar cell based on PffBT3T-E,A: SF-PDI2 exhibits 9.5 % and nearly 90 % internal quantum efficiency despite a low voltage loss of 0.55 V. Our work presents the first example of efficient charge separation upon a small driving force, which will have important fundamental impacts on developing more efficient OSCs.

Welcome to attend the lecture!